This invention relates to a smoke-detecting fire alarm and, more particularly, to a battery-powered fire alarm of this type and a battery voltage monitoring circuit therefor.
Smoke-sensing fire alarms which sense the change in impedance of an ionization chamber when smoke is introduced thereto are well known. Typically, an open ionization chamber is connected in a voltage divider circuit across a power source and a change in impedance is reflected in a voltage change thereacross. This sensing voltage is monitored by a detection circuit and when it exceeds a preselected alarm level, the detection circuit energizes a suitable alarm circuit. In self-contained, i.e., battery-powered, fire alarms of this type, it is known to provide a battery monitoring circuit which will cause a low battery signal to be generated when the battery has been depleted beneath a level at which successful operation of the alarm circuit is assured.
Known detection circuits have suffered from one or more disadvantageous characteristics. For example, in U.S. Pat. No. 3,688,119 of Kobayashi, issued Aug. 29, 1972, to Nittan Company, Limited, a sensing voltage developed between an open and a closed ionization chamber connected in series across a power supply is applied through an impedance matching field-effect transistor amplifier across a Zener diode. An alarm is sounded when the sensing voltage exceeds the breakover voltage of the Zener diode. Disadvantageously, the field-effect transistor operates in its linear range, and a constant current drain is thereby imposed on the power supply. Stricter tolerance on the Zener diode is required, for an alarm voltage is established solely by the Zener diode breakover characteristic. A further problem is that because the actual alarm voltage applied across the Zener diode is unaltered by supply voltage change while the sensing voltage does change with supply voltage, sensing level stability requires regulation of the power supply.
In U.S. Pat. No. 3,714,641 of Scheidweiler, issued Jan. 30, 1973, to Cerberus AG, an alarm circuit is shown in which the current drain problem is overcome by employing a normally off field-effect transistor which turns on only when a sensing voltage developed across an ionization chamber exceeds its threshold voltage. However, the threshold voltage cannot be adjusted and this again requires strict selection of the field-effect transistors with respect to their threshold voltage characteristics, and means for varying sensitivity are lacking. For the same reasons as in the alarm circuit of Kobayashi noted above, the detection circuit requires a stable or regulated supply voltage for successful operation.
Other circuits which enable use of active elements in the detection circuits thereof of widely varying characteristics, or which was adjustable with respect to sensitivity, lack supply voltage stability and consume an unacceptable amount of standby power.
Known battery monitoring circuits also suffer from the problem that they substantially hasten the depletion of the battery which they monitor. This is due to the fact that most monitoring circuits compare the battery voltage to a reference voltage established by a reverse-biased or forward-biased PN junction which requires a significant amount of current. However, battery monitoring circuits are shown in an article entitled "The Lambda Diode: A Versatile Negative-Resistance Device" by Gota Kano et al, appearing at pages 105-109 in the June 26, 1975, issue of Electronics, which overcome the standby power drain problem. The circuits shown therein employ two junction field-effect transistors connected to form what is termed a lambda diode. The field-effect transistors of the lambda diode both remain off until the battery voltage imposed threreacross decreases below the sum of the respective pinch-off voltages thereof. Unfortunately, the reference voltage is thereby fixed, and may not be adjusted such that, again, strict tolerances on the characteristics of the device are required, which increases the cost of manufacturing a monitoring circuit employing the lambda diode.